The present invention relates to a technology for electronic circuits; and more particularly, to a differential current driving type data transmission system.
In general, a differential current driving type data transmission system, supplies different currents to two signal lines in open drain ways, so that a receiving end can recover an original signal based on a voltage difference inputted thereto.
However, concerns arise in such a different current driving type data transmission system in that the effects of an interference phenomenon and noises between transmission lines become stronger as R, L, and C components that are parasitic on the transmission line increase. These concerns are led to the serious distortion of a transmission signal and the lengthy transition time of the signal, which contribute to a reduction in the transmission rate. Moreover, in order to recover an original signal from the distorted signal at the receiving end, complex circuits such as an amplifier should be provided additionally. This naturally increases the complexity of the overall circuit, and the error rate of the system itself during the signal recovery gets higher.
Nevertheless, the distortion of a transmission line due to noises is significantly lower in the differential current driving type than in the single current driving type using one signal line to transmit signals. This is because the influence of noise on differential signal lines is almost the same, so the voltage difference lastly inputted to the receiving end is not much affected by noise. On the contrary, interference between transmission lines differentially influences depending on a distance between signal lines, i.e., the closer the signal lines are to each other, the interference is stronger in proportional to the distance therebetween. As such, the voltage difference of a signal lastly inputted to the receiving end is under the influence of the interference effect, which makes very difficult for the receiving end to recover an original signal.
One general way to resolve such a concern is to increase an amount of current to be supplied in consideration of a possible occurrence of signal distortion due to the noise during signal transmission and the interference between signal lines. When more current is supplied, a voltage difference, between two input signals at the receiving end becomes greater, thereby lowering an error rate in the recovery of an original signal. Although an increase in the amount of current supplied to the transmission line can minimize the distortions of signal, this needs more power and in turn may cause another problem such as an increase in Electromagnetic Interference (EMI) between transmission lines.
In view of such a problem, the inventors of the present invention have proposed, in Korean Patent Application No. 2005-34614 filed on Apr. 26, 2005, a scheme for reducing an error rate in the recovery of an original signal at the receiving end, without necessarily supplying more current to the transmission line as in the existing differential current driving type data transmission system.
The conventional differential current driving type data transmission system includes a transmission section 100, a transmission line 200, and a receiving section 300, as shown in FIG. 1 (see Korean Patent Application No. 2005-34614).
The transmission section 100 includes a line drive controller for outputting differential transmission signals D+ and D− and a line control signal NT, in response to a transmission signal D; and a line driver for driving transmission lines TX+ and TX− with an excitation current source Idc and a base current source Icc, in response to an output signal from the line drive controller.
The receiving section 300 includes an I-V converter for converting transmission currents Irx+ and Irx− that are transmitted through the transmission line 200 into voltage signals Vd+ and Vd−; and a comparator for comparing the voltage signals Vd+ and Vd− to recover an original signal Dr therefrom.
FIG. 2 shows a timing diagram of signals used in the line drive controller of FIG. 1. A line control signal NT in FIG. 2 detects the transition of a transmission signal D such that it becomes inactive in a half period of the unit pulse width of the transmission signal D and becomes active in the other half period.
FIG. 3 illustrates a circuit diagram of the line driver of FIG. 1.
Referring to FIG. 3, the line driver includes transistors TR-1, TR2, TR3 and TR4 for and generating an excitation current source Idc and a base current source Icc by mirroring a reference current Iref; transmission gates TR5, TR6, TR7 and TR8 constituting a first switch SW0 for selectively switching the excitation current source Idc to differential transmission lines TX+ and TX−, in response to differential transmission signals D+ and D−; and transmission gates TR9 and TR10 constituting a second switch SW1 for equalizing the differential transmission lines TX+ and TX− within a common mode interval, in response to line control signals NT+ and NT−.
FIG. 4 depicts a timing diagram of signals used in the line driver in FIG. 3, in which the line control signal NT is inactive in a half period (PT/2) of the unit pulse width PT of the transmission signal D and stays in active until the next transition point of the transmission signal D, such that the differential transmission lines TX+ and TX− in the first half period are driven at Icc, Icc+ and Idc levels depending on the polarity of the transmission signal D and they are equalized at the (2Icc+Idc)/2 level until the next transition point of the transmission signal D, i.e., during the common mode interval.
FIG. 5 illustrates a circuit diagram of the I-V converter in FIG. 1.
Referring to FIG. 5, the I-V converter includes current mirrors 42 and 44 for generating mirroring currents MxIRx+ and MxIrx− by mirroring the transmission currents Irx+ and Irx− flowing through, the transmission lines TX+ and TX−; and voltage dividers 46 and 48 for outputting voltage signals Vd+ and Vd− corresponding to the mirroring currents MxIRx+ and MxIrx− according to resistance ratio of resistors R0 and R1.
As noted before, in the conventional differential current driving type data transmission system, the transmission section 100 provides a common mode interval where the differential transmission lines TX+ and TX− are equalized at the same drive current level. In this case, since the differential transmission lines TX+ and TX− are connected by a switch, they are almost equally influenced by external noise or the interference between signal lines. Further, since the transition of the signal occurs at an intermediate level thereof, signal transition time is relatively shorter than the conventional driving type, thereby increasing the transmission rate.
In the conventional differential current driving type data transmission system, however, when the differential transmission lines TX+ and TX− are equalized at the same drive current level in the common mode interval, self-fluctuation occurs by the inductance component that is parasitic on the signal line itself, and such fluctuation can be readily changed into a differential voltage, which results in distortions in the transmission signal.
In addition, the I-V converting circuit of the receiving section 300 multiplies an applied current using the current mirror, and converts this multiplied current into a voltage. Unfortunately though, the fluctuation of the signal line itself sometimes causes an error during the current-to-voltage conversion. Moreover, it is another problem that the current mirror circuit alone consumes much current.